DDC stands for Direct Digital Control and is the automated control of a condition or process by a digital device. DDC is often used to control HVAC (heating, ventilating, and air conditioning) devices such as valves via microprocessors using software to perform the control logic. This is very similar to a PLC (Programmable Logic Controller). Both are normally free programmable and both communicate with other devices via a communication bus system.Easy speaking you can say that the DDC is for the Building Automation what the PLC is for the Industry Automation. DDC controllers for HVAC are normally delivered with specific application libraries for HVAC control. Also they often provide an open bus system, e.g. like BACnet (Data Communication Protocol for Building Automation and Control Networks).The Desigo Building Automation System is the main Siemens range for HVAC and Building Automation. You can find the “Desigo PX - Automation system for HVAC and building services” – System description under the following link:http://www.hqs.sbt.siemens.com/gip/general/dlc/dlc_get_singlefile.asp?soi=A6V10361879